Offshore structure

ABSTRACT

An offshore structure comprises: an original structure comprising a main platform supported via a foundation on a seabed; and an extension structure comprising a platform extension positioned laterally of the main platform and a platform extension support, depending downwardly from the platform extension, into contact with the foundation, so as to support the extension structure directly on the foundation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 U.S. National Stage of InternationalApplication No. PCT/GB2013/050911 filed on Apr. 9, 2013, which claimspriority to British Patent Application No. 1206377.2, filed on Apr. 11,2012. The contents of the above applications are incorporated herein byreference in their entirety.

FIELD

The present invention relates to an offshore structure, for example anoil and or gas platform, and particularly to how available platform ordeck space on such a structure can be extended.

BACKGROUND

Offshore structures, such as oil and or gas platforms, are common. Oiland or gas platforms are designed to perform a variety of functions,such as drilling wells, extracting oil or natural gas and storing andperforming preliminary processing on any oil or natural gas that isrecovered. In order to perform these various processes, an offshoreplatform must be able to house all the necessary facilities to performthose processes. However, in view of the engineering challenges and costassociated with building a structure that can withstand the naturalenvironmental conditions, such as forces imparted by sea waves, andaccidents such as collisions with moving vessels, offshore structuresare usually designed to be as compact as possible. As such, thestructures are designed to make as efficient use of space as possible.

Nonetheless, it sometimes becomes necessary for an offshore structure toperform new functions, for example due to changes in technology or dueto engineering challenges in extracting resources as they becomedepleted. In such cases it may become necessary to deploy new machineryand/or facilities on the existing structure. In some cases this ispossible by replacing existing facilities. However, that is not alwaysthe case.

Therefore, it sometimes becomes necessary to provide more space on anoffshore structure. One option is to create further decks or levels onthe existing structure. Another option is to extend the platformsideways using cantilevered sections from the side of the existingstructure. Another option is to create an entire new offshore structurein the vicinity of the first structure, and then to link the twostructures together by a bridge, for example.

However, there are various disadvantages associated with the existingmethods of extending offshore structures. Modifying an existingstructure, by introducing new decks, strengthening existing decks, oradding cantilevered sections to the side of the structure can interferewith the ongoing functions of the offshore structure (such as drillingor extraction processes) requiring the structure to be taken offlinewhile the upgrades are made. Further, such modifications can introduceincreases in weight which must be borne by the existing structure, andthis can be impractical in some circumstances. Creating a separate,second, structure, can also be impractical if the sea floor is alreadycrowded with equipment and pipelines in the region of the firstplatform. Further, commissioning an entire new structure is expensive.

As an example of a previous method of extending an offshore structure,GB-B-1,525,242 discloses a conventional cantilever extension structure,supported by struts extending to the legs of the original structure,above the original foundation. As such the extension is supported by thelegs, not the foundation. As another example, GB-A-2,203,782 disclosesan extension structure built as a separate structure on a new foundation(different from the foundation of the original structure).

The present invention provides an alternative way of extending anexisting offshore structure, and aims to at least partly solve some ofthe above-problems.

SUMMARY

According to an aspect of the present disclosure there is provided anoffshore structure comprising: an original structure comprising a mainplatform supported via a foundation on a seabed; and an extensionstructure comprising a platform extension positioned laterally of themain platform; and a platform extension support, depending downwardlyfrom the platform extension into contact with the foundation, so as tosupport the platform extension directly on the foundation.

According to this aspect, the present disclosure provides an extendedoffshore structure. The original structure is original compared to theextension structure. As such, the original structure may have beenconstructed as separate parts, or may have been previously extended. Theoverall extended structure can be constructed with minimal interferencewith the operation of the existing structure, because the extension doesnot interfere with the platform of the existing structure. Further, theplatform extension support depends downwardly, optionally substantiallyvertically, into contact with the foundation of the original structure.As such by utilizing the existing foundation of the original structure,minimal under-water preparation is required. Even if some preparation isrequired, the amount of work is minimal, especially when compared tobuilding an entirely stand-alone extension (linked via a bridge to theoriginal structure, for example). Further, in comparison to acantilevered extension, for example, the weight of the extension istransmitted directly from the extension to the foundation, and not viathe existing structure. As such, the need to reinforce the existingstructure to allow it to bear the weight of the extension and associatedequipment is reduced. Therefore, this construction is very advantageousin situations where there is spare loading capacity in the foundation ofthe original structure, particularly when the main platform of theoriginal structure does not completely overhang the foundations.

The foundation can comprise piles driven into the seabed. The extensionstructure can be directly supported on at least one of the piles, and inone arrangement can be directly supported on only one piling.Preferably, at least one pile has a machined upper surface, and/or hasan open upper end. A bottom end of the platform extension support canfit into the open upper end of the piling. As such, the foundationsprovide a ready-made receiving point, in the form of the pilings, forsupporting an extension. If the machined piling head or other machinedupper surfaces have been removed, alternative connections are possible.

A flange can be positioned above the bottom end of the platformextension support, wherein the flange has an outer width larger than awidth of the hollow upper end diameter of the piling. This ensures theextension is positioned securely.

A bottom end of the platform extension support can comprise asubstantially conical point.

The extension structure can be a monocolumn structure. The monocolumnstructure can depend substantially vertically downwardly from theextension platform. In this arrangement, the centre of gravity of theextension can be balanced over the foundation, and therefore the needfor intermediate lateral bracing is reduced.

The monocolumn structure can taper in width at a lower end, in order tofit within a pile for example. By tapering the end of the platformextension support, it becomes easier to locate and centralize the end inthe appropriate position on the foundation.

The offshore structure can further comprise a lateral brace connectingthe extension structure to the original structure. This provides lateralstabilization of the extension structure. The brace can be in the formof truss, such as a two-dimensional truss. The truss can be connected tothe extension structure by at least one bearing allowing both rotationaland translational motion of the extension structure relative to thetruss. This at least partially reduces the transmittal of any torsionalforces or out of plane stresses into the truss as the extensionstructure moves in response to environmental loads or forces such aswaves. The at least one bearing can comprise a bushing attached to theextension structure, and the truss can be connected by two of saidbearings, and wherein the bushings of each bearing are substantiallyparallel to each other to allow relative vertical movement of themonocolumn (22) to the truss (50). Alternatively the truss could be inthe form of a three-dimensional design.

According to another aspect of the present disclosure, there is provideda kit for extending an offshore structure, the offshore structure beingsupported via a foundation on a seabed, the kit comprising: an extensionstructure comprising a platform extension positionable laterally of amain platform of the offshore structure; and a platform extensionsupport depending downwardly, in use, from the platform extension so asto come into contact with a foundation of the main platform, to supportthe platform extension directly on the foundations of the offshorestructure.

According to another aspect of the present disclosure, there is provideda method of extending an offshore structure, the offshore structurecomprising a main platform supported via a foundation on a seabed, themethod comprising: providing an extension structure, comprising aplatform extension and a platform extension support; positioning theplatform extension laterally of the main platform; and positioning theplatform extension support, depending downwardly from the platformextension, in contact with the foundation so as to support the platformextension directly on the foundation.

According to another aspect of the present disclosure, there is provideda bearing for permitting rotational and translational movement, thebearing comprising: an outer bearing element; an inner bearing elementhaving an inner bearing surface and an outer bearing surface; and abushing; wherein the inner bearing element is provided within the outerbearing element and bears against the outer bearing element along theouter bearing surface, such that the inner bearing element is free torotate in at least two orthogonal directions; and wherein the bushingextends through the inner bearing element and the outer bearing element,such that the inner surface of the inner bearing element bears againstthe bushing and can undergo translation motional with respect to thebushing.

According to this aspect, a bearing that allows both rotation andtranslational movement between, for example, an offshore extensionstructure and a brace to an original structure. This reduces thetransmission of torsional forces and stresses into the brace andoriginal structure, and thereby helps reduce fatigue. As such, thebearing is well suited to use in offshore activities.

The outer bearing surface can be substantially the surface of aspherical segment. That is, the surface can be part of a sphericalsurface. The inner bearing surface can be substantially cylindrical.This allows a high degree of freedom of movement in the bearing.

The bearing can further comprise a housing providing a seal around theinner bearing member, outer bearing member and the outer surface of thebushing. The housing comprises a flexible section to allow the bearingto move, in use. The flexible section can, for example, be in the formof a bellows, allowing the housing to stretch and change shape as thebearing moves. These seals protect the bearing assembly fromenvironmental contamination.

The bearing can further comprise a lip seal between the inner bearingmember and the outer bearing member, and/or a flexible bellows housinglip seal between the inner bearing member and the bushing. This helpsisolate the bearing surfaces from dirt or other substances that mightincrease the friction on the bearing surfaces, and therefore reduce theefficacy and life of the bearing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described below, by way of example only, withreference to the accompanying Figures, in which:

FIG. 1 is a drawing of an offshore structure, incorporating an originalstructure and an extension;

FIG. 2 is a drawing showing how the extension and original structure ofFIG. 1 interconnect;

FIG. 3 is a schematic cross-section view of an alternative arrangementfor connecting an extension and original structure;

FIG. 4 is a drawing of an extension connected to a truss section thatextends from the original structure;

FIG. 5 is a close-up view of the connection between the extension andthe truss section of FIG. 4;

FIG. 6 is a schematic representation of the bearing joint between thetruss section and extension of FIGS. 4 and 5; and

FIG. 7 is a more detailed schematic cross-section of the left-hand sideof the bearing joint of FIG. 6.

DETAILED DESCRIPTION

As discussed above, the present invention is concerned with providing anew way of extending existing offshore structures.

FIG. 1 shows a perspective view of an offshore structure (1), whichcomprises an original structure (10). The original structure (10) is astructure that is fixed to the sea bed (30). The original structure (10)comprises a main platform or deck section (11), which is supported abovethe sea surface. Although not shown in detail in FIG. 1, the mainplatform (11) will usually support equipment necessary for performingthe normal functions of the offshore structure (1).

The main platform (11) is supported via a foundation (40) on the seabed.In FIG. 1, the main platform (11) is supported by the jacket section(12). Typically, such jacket sections (12) can be constructed from steelsections welded together, and are made from tubular sections that arecross-braced against each other. As can be seen in FIG. 1, the jacketsection (12) may comprise leg sections (13) which extend from the seafloor (30) to the deck section (11). These legs (13) are thenstrengthened by the use of braces (14).

The jacket section (12) includes the foundations (40). Foundations (40)typically comprise piles (41) (see FIG. 2) which are driven into the seafloor through steel tubes (42) formed in the foundations (40).

Pilings (41) are typically steel tubes that are driven into the seafloor. The pilings (41) typically have a reinforced, machined, top endthat has a thicker tube wall (for example 100 mm instead of 70 mm) forthe purpose of driving the piling down into the sea floor.

In some cases, pilings (41) are not driven all the way into the seafloor, but are driven to the point at which they can be driven nofurther or have achieved the required penetration per hammer blows tomeet designed capacity. In that case, the unused length of piling (41),not driven into the foundations, are usually cut short above thefoundations, so that the free end of the pile does not extend asignificant distance away from the foundations. However, if a piling(41) is driven to its full depth, no such cutting of the piling isrequired, and the machined top end is left intact. In either case, thepiling (41) will usually have a hollow upper end.

FIG. 1 also shows an extension structure (20). Extension (20) is in theform of a mono column (22) and a module (21). The module is a form ofadditional platform section (21) above sea level. The platform extension(21) is positioned laterally of the main platform (11). It is notnecessary for the platform extension (21) to be positioned at the sameheight as the main platform (11), although this may be advantageous insome circumstances. Further, the platform extension (21) may abut oreven overhang the main platform (11) in some circumstances.

The extension (20) has an extension supporting section (22) extendingdownwardly from the extension platform section (21) and towards thefoundations (40) of the original structure (10). The supporting section(22) extends into contact with the foundations (40), so that theextension (20) is supported directly on the foundations (40). That is,the foundation (40) bears the weight of the extension (20). Further, thefoundation (40) bears the weight of the extension (20) directly, and notthrough the jacket (12) of the main structure, for example, in contrastto a cantilevered extension from a jacket section.

As such, although stand-alone monocolumn platforms as such are known,the extension (20) differs from those conventional columns, because itdoes not have its own supporting base. Further, such conventionalstructures are typically used in shallow water (e.g. about 50 m) and soproviding lateral intermediate stabilisation truss (50) is not normallya major concern. In contrast, in the present invention, the monocolumnextension can be used in waters up to 140 m in depth or even more.

The monocolumn (22) is further connected to the original structure (10)via a truss section (50), which acts as a lateral brace for themonocolumn (22). The truss section (50) is above the sea surface. Thetruss section (50) provides a stabilising connection to the originalstructure, to hold the monocolumn (22) in place. The connection of thetruss (50) to the monocolumn (22) is discussed in more detail below.

An upper section of the monocolumn (22) comprises a lattice trussconstruction around the central monocolumn core. The monocolumn corealso reduces in width within this section. This reduces theenvironmental dynamic loading from the monocolumn (22) onto the mainstructure (10) via the truss (50) by providing a reduced surface areafor waves to impact against. This reduces the force that any waves canapply to the monocolumn (22), whilst ensuring that the extension remainsstrong (to protect it, for example, against attendant vessel impacts).This arrangement therefore mitigates against fatigue occurring in theconnections between the jacket (12) and truss (50) and the truss (50)and monocolumn (22).

FIG. 2 shows a close up of how the bottom of the monocolumn (22) issupported directly on the foundations (40) and pile(s) (41). The bottomof the monocolumn (22) comprises a tapered section (23). The taperedsection (23) is substantially cone shaped, and can, for example, form atwo-step cone such as shown in FIG. 2. In FIG. 2, the step in the coneoccurs below the flange (24) (discussed further below). It is notimportant for the tapered section (23) to be perfectly cone shaped. Thepurpose of the tapered section (23) is to reduce the width of theextension support (22) to assist in locating and centralising the monocolumn (22) in the foundations (40).

In FIG. 2, the monocolumn (22) is supported on the foundations (40) bylocating the tapered section (23) within an upper end of the pile (41).Pile (41) has been driven into the tube structure (pile guides) (42) ofthe foundations (40). As such, the upper end of pile (41) is themachined end provided for driving the pile into the sea floor. Ifhowever, the pile had been cut after it was driven into the sea floor(for example, because the pile (41) was not driven to its full depth),the upper end of the pile (41) can be re-machined in situ, in order toprovide a suitable hollow or socket for receiving the tapered section(23) of the monocolumn (22).

The bottom portion of the monocolumn (22) further comprises a steelflange (24) around the tapered section (23). The steel flange can befully welded around the conical section of the monocolumn. The flange(24) provides a surface that can abut against the top of the pile (41),further assisting in locating the monocolumn into the foundations andhelping in the transfer of load from the module(21) and the monocolumn(22) into the foundation piles (41). The flange (24) can have a width(usually a diameter) that is larger than the outer width (usually adiameter) of the pile (41) in which the extension section (22) islocated. This ensures that that the extension (20) is located assecurely as possible.

Although FIG. 2 shows the monocolumn (22) fitting into a single pile(41) of the foundations (40), it may be desirable to support themonocolumn (22) on more than a single pile. This may be the case, forexample, if there is not enough spare capacity in a single pile tosupport the extra weight of the extension. In that case, as shownschematically in FIG. 3, the bottom of the extension support (22) maycomprise a cross beam (25) on which the main monocolumn (22) issupported, and which connects to multiple (two, in the case of FIG. 3)tapering sections (23) that can fit into piles (41) of the foundations(40). In such cases it may be necessary to prepare the foundationssuitably, so that the load of the extension can be evenly supportedacross the piles (41).

FIG. 4 shows the monocolumn (22) (without the platform extension section(21)) attached to a brace, in the form of truss section (50). The trusssection (50) extends to connect to two jacket legs (13). The trusssection (50) comprises two main chord members (51), which are internallybraced by brace sections (52). As such, the truss section (50) is a“two-dimensional” truss section, (although three-dimensional trussoptions are possible) meaning that the bracing sections (52) are withinthe plane between the two chord members (51). In general, any form ofbrace can be used, and the brace may connect to any part of theextension (20), such as the module (21) or the monocolumn (22).

In FIG. 4, the truss section (50) is rigidly attached (for example bywelding) to the jacket legs (13). Bolting and grouting are alternativeoptions for making the attachment. However, a different connection ismade between the truss section (50) and the monocolumn extension (20).

FIG. 5 shows a close-up drawing of the upper portion of the monocolumn(22) of FIG. 4. As can be seen in FIG. 5, the truss section (50) isattached to the monocolumn (22) via a bearing joint (60) which acts ajoint at the end of each chord (51). The bearing joints (60) are rigidlyconnected to the brace section, and in this case are welded to thechords (50). Further, the bearing joints (60) are set over support postsof the monocolumn (22), in a manner which allows both rotational andvertical movement between the monocolumn (22) and the truss section(50). This essentially creates a pin connection between the truss (50)and the monocolumn (22), and so isolates the truss from torsion andbending forces due to the motion of the monocolumn (22) relative tooriginal structure (10).

Even though the extension (20) and original structure (10) share thesame foundations, the monocolumn (22) and the original structure (10)will be subject to differing forces, due to waves or vessel impact forexample. As such, the monocolumn (22) may twist and/or bend relative tothe original structure (10).

The bearing joint (60) is shown in more detail in FIG. 6. Support post(26) of the monocolumn extension (20) is surrounded by cylindricalbushing (63). The bushing (63) is attached to the support column (26)via grouting (64), such that the bushing is held substantiallymotionless with respect to the support column (26).

The grouting (64) between the bushing (63) and the support column (26)allows for some adjustment when positioning the bushing (63) on thesupport column (26). This is helpful when there are multiple bearingjoints (60), as it allows the cylindrical bushings to be alignedsubstantially parallel with each other, thus facilitating the samemovements occurring in the individual bearings at the same time.

A spherical bearing, comprising an outer bearing member (61) and aninner bearing (62) are positioned around the bushing (63). The innersurface of the inner bearing member (62) bears against the outer surfaceof the cylindrical bushing (63). The inner surface of the inner bearingelement (62) is preferably of cylindrical shape. The inner bearingelement (62) is thus able to move parallel to the cylindrical axis ofthe bushing (63) (i.e. moving from one end of the bushing (63) to theother) and is also able to rotate around the bushing (63) (i.e. within aplane perpendicular to the cylindrical axis of the bushing (63). Thebearing surfaces are lubricated in order to provide very low frictionsurfaces. The surfaces are the inner surface of the inner bearingelement (62) and the outer surface of the bushing (63). Similarly, it isdesirable to lubricate the outer surface of the inner bearing element(62) and the inner surface of the outer bearing element (61).Lubrication in the form of oil, grease, PTFE impregnated tape, forexample, can be used.

The outer surface of the inner bearing element (62) is curved. The outersurface of the inner bearing element (62) bears against the outerbearing element (61). The outer bearing element has a bearing surfacecomplimentary in shape to the outer bearing surface of the inner bearingelement (62). Preferably, the outer bearing surface of the inner bearingelement (62) is convex, and the bearing surface of the outer bearingelement (61) is concave. Preferably, the outer bearing surface of theinner bearing element (62) is the surface of a spherical segment. Thatis, preferably the outer bearing surface of the inner bearing element(62) forms part of the surface of a sphere. This allows the greatestfreedom of motion between the inner and outer bearing elements (61) and(62).

The spherical bearing (61) (62) allows for rotational movement betweenthe truss section and the support column (26). As such, the combinationof the spherical bearing and the bearing along the bushing (63) allowsboth rotational and translational movement between the truss (50) andthe monocolumn (22), thus minimizing the transmission of any torsionalstresses into the truss (50) due to motion of the monocolumn (22).

The bearing elements (61) and (62) are housed within a housingcomprising elements (65) and (66). Housing elements (65) and (66) aredesigned to slide together in a wedge formation in order to assist inpositioning the bearing joint on the support column (26).

Due to the harsh environmental conditions experienced on offshorestructures, the bearing joint (60) incorporates several measures inorder to ensure that the bearing surfaces are kept clean. FIG. 7 shows aclose up of one side of the bearing. As can be seen, housing elements(67) extend over and under the outer bearing element (61), towards thebushing (63). However, the housing elements (67) themselves do not touchthe bushing (63). A lip seal (68) positioned at the end of each housingelement (67) near the bushing (63) extends to contact the face of thebushing. The lip seal may be energized by incorporating a ring spring,to bias the seal against the bushing. Further, a void (69) is createdbetween the lip seal (68) and the inner bearing element (62). This voidcan be filled with grease in order to assist in the prevention ofcontaminates reaching the low friction bearing surfaces between theinner bearing element (62) and the bushing (63), and also the innerbearing element (62) and the outer bearing element (61).

A further lip seal (70) is present at the outer edges of the innerbearing surface of the inner bearing element (62). Once again, this lipseal provides an additional barrier to any dirt or contaminants, thusallowing the bearing surface (62) to move as freely as possible over thebushing (63).

Similarly, measures are taken to maintain the integrity of the bearingsurface between the inner bearing member (62) and the outer bearingmember (61). A protective packing (71) is positioned between the housingelement (67) and the bearing element (61) and (62). Protective packing(71) is positioned at the outer interface of the bearing surface betweenthe two bearing elements, and forms a physical barrier to prevent anycontaminants entering between the two bearing elements. Additionally, anO-ring (72) is positioned towards the edge of the bearing surface of theouter bearing element, to provide an additional barrier to anycontaminants.

In addition to these precautions, taken close to the individual bearingfaces, the housing is further sealed by a flexible rubber bellowsmaterial (73), thus providing an overall seal between the outermosthousing element (60) and the support column (26). The flexible nature ofthe bellows seal (73) allows for the seal to be maintained even as theextension (20) and the truss (50) move relative to each (i.e. even asthe bearing joints (60) move).

Either or both of the lip seals (68) and (70) may be biased towards thebushing (63) by the use of a ring spring, for example.

In use, the monocolumn (22) is vertically supported on the foundations(40) of the original structure (10). The monocolumn (22) directlycontacts the foundation (40). That is, the monocolumn (22) rests on thefoundation (40). The truss (50) provides a means for transferring anylateral loads from the monocolumn (22) (i.e. induced by wave motion) tothe jacket (12) of the original structure, thus providing lateralsupport for the monocolumn (22). Because the truss (50) is rigidly fixedto the jacket (12) (for example by welding) the possibility of stressesbeing induced in the truss (50) due to vertical forces/displacement atthe monocolumn connection is minimized by the use of the bearing joints(60), which effectively form pin connections that are moment released.Monitoring of the bearing performance and the torsional loads in thetruss are monitored by means of suitably positioned strain gauges andrecording and monitoring system to detect any increased torsionalloading into the truss should problems with the bearings occur. Thisallows preventative maintenance to be undertaken rather than shuttingdown the platform when fatal problems occur.

To further ensure that the pinned bearing joints (60) are free to movevertically, it may be desirable to support the weight of the truss (50)using additional hangers from the original structure (10).

The monocolumn (22) may be installed in place by a heavy lift vessel,for example. Once the extension platform has been positioned laterallyof the original structure, vertical support can be provided whilst theextension is lowered and located into the pile (41) of the foundations(40) whilst lateral support can be provided by arms extending from thetruss (50). As the monocolumn is installed, the truss can further beutilized to mount equipment to assist in the aligning of the column intothe correct vertical alignment with respect to the platform, once thecolumn is supported on the pile. As such, the monocolumn extension canbe provided as a kit for extending an existing offshore structure.

Although the above description has focused on the provision of anextension (20) comprising a monocolumn (22), the concept of utilizing afoundation of an existing offshore structure to directly support anextension is applicable to any form of extension. For example, theextension could have two supporting legs that each supported on adifferent section of the foundation. Further, the extension supportsection need not be straight (although this gives benefits in terms ofbalance) and could incorporate dog-legs for example. Any alteration ofthe centre of gravity could be compensated for by attaching theextension to the original structure at higher points, to absorb thelateral load.

The above description is provided by way of example only, and is notintended to be limiting upon the scope of the invention. The scope ofthe invention is defined in the appended claims.

The invention claimed is:
 1. An offshore structure comprising: anoriginal structure comprising a main platform supported via a foundationon a seabed; an extension structure comprising a platform extensionpositioned laterally of the main platform and a platform extensionsupport, depending downwardly from the platform extension, into contactwith the foundation, so as to support the extension structure directlyon the foundation; and a lateral brace connecting the extensionstructure to the original structure, wherein the brace is connected tothe extension structure by at least one bearing allowing both rotationaland translational motion of the extension structure relative to thebrace.
 2. The offshore structure according to claim 1, furthercomprising a flange positioned above the bottom end of the platformextension support, wherein the flange has an outer width larger than awidth of the hollow upper end of the piling.
 3. The offshore structureaccording to claim 1, wherein the bottom end of the platform extensionsupport comprises a substantially conical point.
 4. The offshorestructure according to claim 1, wherein the extension structure is amonocolumn structure.
 5. The offshore structure according to claim 4,wherein the monocolumn structure depends substantially verticallydownwardly from the extension platform.
 6. The offshore structureaccording to claim 4, wherein the monocolumn structure tapers in widthat a lower end.
 7. The offshore structure according to claim 1, furthercomprising a lateral brace connecting the extension structure to theoriginal structure.
 8. The offshore structure according to claim 7,wherein the brace is in the form of a truss.
 9. The offshore structureaccording to claim 8, wherein the truss is a two-dimensional truss. 10.The offshore structure according to claim 1, wherein the at least onebearing comprises a bushing attached to the extension structure.
 11. Theoffshore structure according to claim 10, wherein the brace is connectedby two of said bearings, and wherein the bushings of each bearing aresubstantially parallel to each other.
 12. The offshore structureaccording to claim 1, wherein the bearing includes: an outer bearingelement; an inner bearing element having an inner bearing surface and anouter bearing surface; and a bushing, wherein the inner bearing elementis provided within the outer bearing element and bears against the outerbearing element along the outer bearing surface, such that the innerbearing element is free to rotate in at least two orthogonal directions,and wherein the bushing extends through the inner bearing element andthe outer bearing element, such that the inner surface of the innerbearing element bears against the bushing on the outer surface of thebushing and can undergo translation motional with respect to thebushing.
 13. The offshore structure according to claim 1, wherein thefoundation comprises pilings driven into the seabed, and wherein abottom end of the platform extension support fits into a hollow upperend of the piling.
 14. The offshore structure according to claim 13,wherein the extension structure is directly supported on more than oneof the pilings.
 15. The offshore structure according to claim 13,wherein the extension structure is directly supported on only onepiling.
 16. A kit for extending an offshore structure, the offshorestructure being supported via a foundation having pilings driven into aseabed, the kit comprising: an extension structure comprising a platformextension positionable laterally of a main platform of the offshorestructure; a platform extension support depending downwardly, in use,from the platform extension so as to come into contact with at least oneof the pilings of the foundation of the main platform, to support theplatform extension directly on the foundation of the offshore structure;and a lateral brace connecting the extension structure to the originalstructure, wherein the brace is connected to the extension structure byat least one bearing allowing both rotational and translational motionof the extension structure relative to the brace.
 17. The kit accordingto claim 16, wherein a bottom end of the platform extension support fitsinto a hollow upper end of the piling.
 18. The kit according to claim16, wherein the at least one bearing comprises a bushing attached to theextension structure.
 19. The offshore structure according to claim 18,wherein the brace is connected by two of said bearings, and wherein thebushings of each bearing are substantially parallel to each other.
 20. Amethod of extending an offshore structure, the offshore structurecomprising a main platform supported via a foundation on a seabed, themethod comprising: providing an extension structure, comprising aplatform extension and a platform extension support; positioning theplatform extension laterally of the main platform; positioning theplatform extension support, depending downwardly from the platformextension, in contact with the foundation so as to support the platformextension directly on the foundation; and connecting a lateral brace tothe extension structure by at least one bearing allowing both rotationaland translational motion of the extension structure relative to thebrace.
 21. The method according to claim 20, wherein positioning theplatform extension support in contact with the foundation includespositioning a bottom end of the platform extension support into a hollowupper end of a piling of the foundation.